Best way to round off the edges in a rectangle
If i understood the question

Shift+A Add Plane

Go to edit mode and make the shape rectangular

Come out of the edit mode and Press Ctrl+A Apply rotation &amp; scale

Go to edit mode with the verts selected Press Ctrl+Shift+B Bevel it and scroll to add segments

With the rectangular rounded shape selected. Press I to offset the face

Now select the edge and extrude it and delete the top face. Select the bottom edge and Press F to fill the face